CVE-2016-6581 #56
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CVE-2016-6581 #56
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This also defaults the max_header_list_size to 64kB.
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A HTTP/2 implementation built using the priority library could be targetted for a denial of service attack based on HPACK, specifically a so-called "HPACK Bomb" attack.
This attack occurs when an attacker inserts a header field that is exactly the size of the HPACK dynamic header table into the dynamic header table. The attacker can then send a header block that is simply repeated requests to expand that field in the dynamic table. This can lead to a gigantic compression ratio of 4,096 or better, meaning that 16kB of data can decompress to 64MB of data on the target machine.
It only takes a few such header blocks before the attacker has forced the target to allocate gigabytes of memory, which will take the process down. This requires relatively few resources on the part of the attacker.
While we are not aware of any attacker actively exploiting this vulnerability, it has been public disclosed in this report, and so users should assume that they are likely to be targetted by such an attack.